So it sounds like you're getting the plat from the champ itself which means you have also been eligible for the skull each time you've gotten plat from the champ, but just haven't been lucky enough for it to drop to you yet.
I hate to say this cause I do think dexxers need a little more dev work but I only play an axer and I got a skill from a public riktor. It is possible just very unlikely. Also, Arnold stated it perfectly for harrower. Get close and in the right position and just whack away. A dexxer is extremely unlikely to do the most damage but with all the dragons you should be pretty safe from attack.
For what it's worth, when bosses do drop loot where it is weighted by damage done, only the top pet's damage is counted towards a tamer's damage output - the other pets are omitted from the calculation. If the enemy has tamer damage reduction or the dexer has a slayer, dexers can easily outdamage tamers for loot calculation purposes since they are only competing against one dragon. Tamer-hybrid characters also excel because of this. (eval tamers, dexer/tamer hybrids, etc... this is why Descartes tops so many of the champion leaderboards)
I've gotten the skull without getting plat. So has Zyler. There is almost zero chance it was stolen because I run around at the to avoid that very thing. Someone would have had to random steal it from a moving target before I saw my status bar update. I also tried restarting my client... nada.
seeing as the skull drop is completely fucking random, i have had PoS mages literally run up on a champ, hit it twice with EB's and stolen my skull so idk what ur complaining about other than the RNG ins't giving u the skull. also every champ you can easily EVAL mage and get plat , and arnold is right. All you need is Red pots and seriously . maybe 4-5 and you can easily get in the dragon pile, you just need to know where to stand.
I've been using two melee characters along with my tamer for ages now(for plat purposes), and they've only been dropped a couple times, due to my error. I'm only theorizing on this, but I believe dealing direct damage (yourself, instead of through summons, or pets) has a higher percentage yield in terms of damage share. My reasons for thinking this, is because I -have- had such success with my axers at champs, and because I've lost plat rights on several occasions to axers (on my tamer) to regular bosses such as colossi. Hope this helps!
From my understanding your damage to a boss is graded by the highest of your personal damage or any individual pet damage. So if an axer could beat the damage output of one of the pets or the tamer itself then he'd win (even if the tamer's combined personal and pets damage was higher.)